One of the most common processes in mining and metallurgy is the comminution processing or disintegrating of ore. When processing material for the selective or collective recovery of valuable material components, the processes concerned are preceded by comminution processing i.e. mechanical crushing or disintegration of the material in a manner to free the valuable components, one from the other. Comminution is particle size reduction of materials. Comminution is achieved by blasting, crushing and grinding. After comminution the components are then mutually isolated with the aid of known separation methods, this isolation being contingent on differences in color, shape, density or in differences in their respective surface active and magnetic properties, or other properties.
In comminution processing first ore or rock is excavated, broken down or removed by blasting. Blasting is the controlled use of explosives and other methods in mining, quarrying and civil engineering. Typically blasting produces particles in the size having a diameter of 500 mm or more.
Crushing is particle size reduction of ore or rock materials by using crushing devices i.e. crushers. Crushers e.g. jaw crushers, gyratory crushers or cone crushers are used to reduce the size, or change the form, of materials so that pieces of different composition can be differentiated. In the crushing process the crushing devices hold material being crushed between two parallel or tangent solid surfaces of a stronger material and apply sufficient force to bring said surfaces together. Typically in a crushing process particles having a diameter up to 1000 mm are crushed to particles having a diameter of 5 mm or more.
Grinding is particle size reduction of ore or rock materials in grinding mills. In hard rock mining and industrial mineral operations the demands for rotating mineral and metallurgical processing equipment such as grinding mills are very high both in terms of grinding efficiency and energy consumption. Typically in a grinding process particles having a diameter up to 1000 mm are grinded to particles having a diameter of 0.010 mm or more. This conventional grinding of materials results in considerable wear on the grinding bodies present in the mill, due to the hardness of the rock concerned, therewith also resulting in considerable costs for the provision of such grinding bodies.
The rotating mineral and metallurgical processing equipment such as grinding mills are typically very large, having a diameter of several meters. The grinding mills may be trunnion-supported or shell-supported. Trunnion support is the most common way of supporting a mill in a mineral processing application, especially in very large grinding mills. In a bearing arrangement of a trunnion-supported grinding mill the support bearings have a relatively small bearing diameter and the trunnion journals have a high consistent stiff journal surfaces, this facilitating the formation of a good bearing lubricant film distribution. The shell-supported grinding mills are more compact, occupy less floor space and require simpler foundations than comparable trunnion-supported grinding mills. Because the end plates of the shell-supported grinding mill do not support the structure, the feed and discharge openings may be sized to meet process conditions without being constrained by trunnion bearing limitations.
A ball mill is a typical type of fine grinder. However, the rotating mineral and metallurgical grinding mills are today very often autogenous grinding mills or semi-autogenous grinding mills designed for grinding or primary crushed ore. Autogenous grinding mills are so-called due to the self-grinding of the ore. In an autogenous grinding mill a rotating drum throws larger rocks of ore in a cascading motion which causes impact breakage of larger rocks and compressive grinding of finer particles. In autogenous grinding the actual material itself, i.e. the material to be ground, forms the grinding bodies.
Semi-autogenous grinding mills are similar to autogenous mills, but utilize grinding balls e.g. steel grinding balls to aid in grinding like in a ball mill. Attrition between grinding balls and ore particles causes grinding of finer particles. Semi-autogenous grinding mills typically use a grinding ball charge of 8 to 21%, sometimes a grinding ball charge of 5 to 60%. A semi-autogenous grinding mill is generally used as a primary or first stage grinding solution. Semi-autogenous grinding mills are primarily used at gold, copper and platinum mines with applications also in the lead, zinc, silver, alumina and nickel industries.
Autogenous and semi-autogenous grinding mills are characterized by their large diameter and short length as compared to ball mills. The rotating mineral and metallurgical processing equipment such as autogenous and semi-autogenous grinding mills are typically driven by ring gears, with a 360° fully enclosing guard.
The inside of an autogenous or semi-autogenous grinding mill is lined with mill linings. The mill lining materials typically include cast steel, cast iron, solid rubber, rubber-steel composites or ceramics. The mill linings include lifters, e.g. lifter bars to lift the material inside the mill, where it then falls off the lifters onto the rest of the ore charge.
Rotating mineral and metallurgical processing equipment that is provided with internal lifters is typically difficult to control. For example, in autogenous grinding mills or semi-autogenous grinding mills the feed to the mill also acts as a grinding media, and changes in the feed have a strong effect on the grinding efficiency. The change in the feed properties is a normal phenomenon that needs to be considered in controlling the rotating mineral and metallurgical processing equipment.
In autogenous or semi-autogenous grinding mills, the existing mineral deposits seldom have a homogenous structure and a homogenous mechanical strength. Material properties such as hardness, particle size, density and ore type also change constantly and consequently a varying energy input is required.
Conventionally grinding has been controlled on the basis of the mill power draw, but particularly in autogenous and semi-autogenous grinding, the power draw is extremely sensitive to changes in feed parameters. It has been discovered that the degree of fullness in the mill as percentages of the mill volume is a quantity that is remarkably more stable and much more descriptive as regards the state of the mill. But because the degree of fullness is difficult to infer in an on-line-measurement, the measurement of the load mass is often considered sufficient. However, the mass measurement has its own problems both in installation and in measurement drift. Moreover, there may be intensive variations in the load density, in which case changes in the mass do not necessarily result from changes in the degree of fullness.
As a summary, the degree of fullness is an important parameter that describes the state of the grinding mill. The main challenge with the degree of the fullness is that the parameter is difficult to measure online. One prior art method for determining the degree of fullness of a large grinding mill drum has been to measure the weight of a large grinding mill drum and use the measured weight to calculate the degree of fullness of a large grinding mill drum. In this prior art method the weight of the grinding charge has been used as the deciding parameter for controlling the mill. This method is cost demanding, however, because of the weighing equipment needed to register continuously the changes in the weight of the grinding charge that occur during operation of the mill, which enables the steps necessary in order to improve prevailing operating conditions to be carried out as quickly as possible. Also the water content of the mill changes constantly, the density, hardness and particle size of the grinding charge changes constantly. Furthermore the mill linings typically constitute up to 30-50% of total weight of the mill. As these linings wear off in time this has a considerable effect on the weight of the mill. Therefore the weight of the grinding mill drum is not a good indication the degree of fullness in the grinding mill drum. All in all it has been discovered that the weight of the grinding charge does not correlate good enough with the degree of fullness in the grinding mill drum as percentages of mill volume.
Another prior art method for determining the degree of fullness of a large grinding mill drum has been to measure and analyze the power consumption or the power intake signal of a large grinding mill and use the measured power consumption to calculate the degree of fullness of a large grinding mill drum. However, particularly in autogenous and semi-autogenous grinding mills, the power consumption is extremely sensitive to changing parameters. The energy or power requirement of a large grinding mill depends on several factors, such as the density of the grinding charge, a mill constant, the extent of mill charge replenishment, or the instant volume of charge in the grinding mill, relative mill speed, length and diameter of the grinding mill. Furthermore, it has been discovered that the grinding mill power consumption or the power intake signal does not correlate enough with the degree of fullness in the grinding mill drum as percentages of mill volume.
The above presented two prior art methods are off-shell-device type methods for determining the degree of fullness of a large grinding mill drum. That is, the measuring devices are installed on the side of the grinding mill on the surrounding structure. A third off-shell-device type prior art method for determining the degree of fullness of a large grinding mill drum has been to measure acoustic wave properties of a large grinding mill and use the measured acoustic wave properties, i.e. sound pressure and/or sound intensity to estimate the degree of fullness of a large grinding mill drum. In the third prior art method the off-shell-device type acoustic wave property measurement sensors may be a single microphone or a series of microphones or microphone mats that are measuring acoustic wave properties coming from the large grinding mill. Also here, it has been discovered that the off-shell-device measured grinding mill acoustic wave properties provide only a rough estimate on the degree of fullness.
In the following, the prior art will be described with reference to the accompanying FIG. 1, which shows a cross-sectional view of a large grinding mill drum according to the prior art.
FIG. 1 shows a cross-sectional view of a large grinding mill drum according to the prior art. In FIG. 1 the grinding mill has a drum casing 1, which drum casing 1 is provided with linings. The linings of the drum casing 1 comprise lifting bars 2, which lifting bars 2 lift the grinding charge material inside the mill, where it then falls off the lifting bars 2 onto the rest of the grinding charge. The angle in which the grinding charge material inside the mill first hits a lifting bar 2 is called “toe angle” φk. Respectively the angle in which the grinding charge material inside the mill first falls off a lifting bar 2 is called “shoulder angle” φs.
Over the recent years there has also been a lot of development around on-mill-shell type of devices. In U.S. Pat. No. 6,874,364 a system for monitoring mechanical waves from a moving machine has been presented in which system a sensor arrangement is located on an exterior surface of the grinding mill drum. The presented sensor arrangement has an acoustic wave sensor for measuring acoustic wave properties and an accelerometer for measuring mechanical waves, i.e. vibrational events and low frequency events, event spatial localization, and events occurring on the ends of the mill. The presented mechanical wave monitoring method may also include a step of monitoring volumetric load in the machine based on the measured mechanical waves. However, even the presented on-mill-shell type device measured grinding mill acoustic wave properties do not correlate adequately enough with the degree of fullness in the grinding mill drum as percentages of mill volume.
In U.S. Pat. No. 5,360,174 an arrangement for registering the instant grinding charge volume of a grinding drum has been presented in which arrangement there is integrated a tension sensor on a flexible bar inside a rubber or steel-cap lifter bar of the grinding mill drum. In the U.S. Pat. No. 5,360,174 patent specification there is in FIG. 1 presented a point A where a lifting device will engage the grinding charge, said point A also commonly referred to a toe position or toe angle. Similarly in FIG. 1 of said patent specification there is presented a point B where a lifting device will leave its engagement with the grinding charge, said point B also commonly referred to a shoulder position or shoulder angle. The tension sensor arrangement presented in the U.S. Pat. No. 5,360,174 patent specification detects a tension on a lifter bar caused by the grinding charge load. However, the presented tension sensor arrangement requires customized lifter bars of the grinding mill drum.
In U.S. Pat. No. 7,699,249 there is presented a method for defining the degree of fullness in a mill is calculated on the basis of the measured toe angle, the rotation speed of the mill and the geometrical dimensions of the mill. However, the presented sensor arrangement does not consistently enough provide straightforward and adequate measurement sensitivity required for a precise monitoring of the degree of fullness in the grinding mill drum as percentages of mill volume.
In general, there are some problems with the prior art solutions for measuring the degree of fullness of a large grinding mill drum. So far, the measuring solutions are relatively complex and difficult in order to provide reliable information. Also the measurement accuracy and reliability with the prior art measuring solutions has not been adequate enough.
The problem therefore is to find a solution for measuring the degree of fullness of a large grinding mill drum which can provide reliable measurement data for the determination of the degree of fullness of a large grinding mill drum with better measurement accuracy and reliability.
There is a demand in the market for a method for determining a degree of fullness of a large grinding mill drum which method would be more reliable and have a better measurement sensitivity when compared to the prior art solutions. Likewise, there is a demand in the market for an arrangement for determining a degree of fullness of a large grinding mill drum which arrangement would be more reliable and have a better measurement sensitivity when compared to the prior art solutions; and also a demand for a large grinding mill drum having such characteristics.